Problem: The scale on a map is 5cm : 4km. If two cities are 15cm apart on the map, what is the actual distance in km between the two cities?
The scale means that every 5cm on the map corresponds to an actual distance of 4km. A distance of 15cm on the map is the same as 3 $\cdot$ 5cm. The actual distance between the two cities is 3 $\cdot$ 4km, or 12km.